1. Field of the Invention
The present invention relates to an optical rotary encoder for acquiring rotation information such as rotation angle/speed of a rotating shaft.
2. Description of the Related Art
Conventionally, there have been known various types of optical encoder.
Japanese Patent Publication No. 2002-48602 discloses an optical rotary encoder that includes a scale with a reflective-type optical pattern or diffraction grating pattern for rotation about an axis or centerline and a substrate on which a surface-emitting light source is located on the axis of rotation of the scale and a plurality of light-detecting elements are arranged in a circumferentially equally spaced relation and around the light source.
Japanese Patent Publication No. 58-147654 discloses an optical rotary encoder in which a plurality of light-detecting elements are arranged circumferentially and entirely around the axis of rotation and located at a side opposite to the light source with regard to a code wheel.
Japanese Patent Publication No. 9-196703 discloses an optical rotary encoder in which a light source is located on the axis of rotation of the code wheel. Light emitted from the light source enters the code wheel and then is reflected through its two inclined planes at a 45 degrees angle with regard to the axis of rotation and then enters a code pattern (slit openings) in a direction perpendicular to the code pattern.
Japanese Patent Publication No. 64-74412 discloses an optical linear encoder in which a light-emitting element and light-detecting elements are located at a side opposite to a reflecting mirror with regard to a scale. The mirror reflects light rays from the light-emitting element so that parallel beam of light is directed toward the light-detecting elements.
U.S. patent application Publication No. 2002/0038848 discloses an optical rotary encoder in which a reflective surface is formed on the code wheel. Light emitted from the light source is reflected through the reflective surface so that the reflected light is parallel to the axis of rotation of the code wheel.
In the optical rotary encoder described in JP 2002-48602 (A), light beam emitted from the light enters the scale so that the principal axis of the beam is perpendicular to the scale. Light beam that enters a region at a radius of r1 on the scale and is reflected therethrough is incident into a region at a radius of r2 on the light-detecting elements. The radii r1 and r2 satisfy an equation (1) r2=r1(z1+z2)/z1 where z1 is a distance between the light source and the scale and z2 is a distance between the scale and the light-detecting elements. Since the thickness of the light source is sufficiently smaller than the distance between the light source and the scale, the equation (1) leads to an approximate equation (2) r2=2×r1.
In gengeral, in order to obtain an optical rotary encoder with high resolution, the number of slit opening of the code pattern on the code wheel need to be increased. Further, in order to reduce the size of the code wheel while the number is maintained, the pitch of the slit openings must be reduced. The limitations on size of the code wheel and therefore of the encoder and on level of resolution are deteermined by the degree to which the pitch of the slit openings can be reduced. In the case of the opitcal rotary encoder in JP 2002-48602 (A), the radial length of the light-emitting elements is twice as that of the code pattern in accordance with the equation (2). In other words, even if the pitch of the slit openings of the code pattern is reduced as possible, the size of the encoder depends upon the size of the light-detecting elements having a radial length twice as large as that of the code pattern, which may result in an insufficient downsizing and level of resolution.
Also, since beam of light which is reflected by the scale and then enters the light-detecting elements is not parallel, if the distance between the light-detecting elements and the scale is varied, an illumination on the scale is greatly changed. As a result, the quanty of light-deteced by the light-detecting elements varies, leading to detection error.
Further, since the scale is of a reflective type, if the scale is inclined, a light pattern projected onto the light-detecting elements changes, resulting in detection error.
In JP 9-196703 (A), although light that enters the code wheel from the light source is parallel beam of light, since the light is reflected through the two inclined planes at a 45 degrees angle with regard to the axis of rotation and then illuminates the code pattern, the variation in orientation of the shaft would cause an illumination distribution on the code pattern to be greatly changed. This also results in detection error. Further, since the width of the beam of light illuminated on the code pattern is about a half as large as the diameter of the parallel beam of light that enters the code wheel, a region where the code pattern can be formed is limited. As a result, downsizing of the encoder would lead to a lower quantity of detected light, thereby limiting its level of resolution.